The present invention relates to a valve unit for controlling the delivery of a combustible gas according to the preamble of the main claim.
It is well known that such units are used for controlling the delivery of combustible gas to a burner or other similar consumer unit so as to vary in a controlled manner its delivery pressure or the flow rate of gas delivered.
Valve units of this type, known from the production of the same Applicant, are typically provided with motor driven actuators for the operating control of a closure means for closing and opening a valve seat provided in the delivery pipe. The actuators comprise, for example, an operating rod acting on the closure means and connected by way of a screw/nut-screw coupling to the rotor of an electric motor in order to displace the closure means for closing and opening of the valve seat as a result of rotational actuation of the electric motor. Through the control of the actuator there is likewise obtained control of modulation of the delivery pressure or respectively of the flow rate of gas delivered.
A problem encountered in valve units with motor driven actuators of the type indicated is that of guaranteeing effective interception of the passage of gas through the valve seat, when predetermined conditions occur, for example those in which safety closure of the valve seat must be ensured as a consequence of the interruption of the electrical supply of the motor driven actuator.
In the modulation phase, interruption of the electrical supply of the motor driven actuator may cause the closure means to stop in an intermediate position of opening of the seat, and therefore the interception of the flow of gas through said seat cannot be guaranteed.
Likewise known are motor driven actuators of reversible type which are brought into the position of closure of the valve seat, on interruption of the electrical supply, by the action of the resilient force of a pre-loaded spring. The closure thus obtained however is typically unreliable and not suitable for safety valves.
The problem underlying the present invention is that of providing a valve unit structurally and functionally designed so as to remedy all the drawbacks mentioned with reference to the prior art cited.
This problem is solved by the invention by means of a valve unit produced in accordance with the claims which follow.